WEDNESDAY, Nov. 22, 2017 (HealthDay News) -- For a quarter of American seniors, a cancer diagnosis signals the return of an old foe, new research shows.

Even in cancer patients younger than 65, one in 10 cases involves people who've had the disease before, the study of nearly 741,000 people found.

Depending on a patient's age or cancer type, the frequency of prior cancers ranged anywhere from about 4 percent to 37 percent, the researchers said, "and most prior cancers were diagnosed in a different cancer site."

The trial was led by Caitlin Murphy of the University of Texas Southwestern Medical Center, Dallas, and published online Nov. 22 in JAMA Oncology.

One oncologist said the findings are a logical consequence of advances in cancer care.

"Unfortunately, this increases the pool of patients eligible to develop new unrelated cancers, or cancers that arise as a long-term consequence of prior cancer therapy," said Smaldone. He is an associate professor of surgical oncology at Fox Chase Cancer Center in Philadelphia.

In the study, Murphy's group looked at 2009-2013 data from national cancer registries, focusing specifically on people newly diagnosed with cancer. The investigators found many were already cancer survivors at the time their new tumor was diagnosed.

"From a patient perspective, having multiple cancer diagnoses complicates and exacerbates already unmet survivorship needs," Smaldone said. For example, even people who've encountered the disease just once can suffer from depression or worries about how to pay for care.

What's more, clinical trials that are exploring potentially helpful new drugs often exclude patients with a prior cancer history, he added.

According to Dr. Stephanie Bernik, chief of surgical oncology at Lenox Hill Hospital in New York City, "Strides need to be taken to ensure that these patients have access to clinical trials, as trials may offer treatments not available to the cancer patients at large."

Reviewing the new findings, she said that certain people -- because of either their DNA or lifestyles -- may be more vulnerable to repeat battles with cancer.

"Because several cancers develop in patients with underlying genetic abnormalities, it is understandable how a patient with one cancer is at risk for a subsequent cancer," Bernik said.

"Many cancers can be part of a syndrome, and risk factors for cancer also overlap," she added. "Smoking and alcohol are both risk factors for several different cancers, so patients that smoke or drink are lowering their threshold for cancer development."